The goal in constructing a precipitation-based measure of ENSO was to estimate the gradient of rainfall anomalies across the Pacific basin and ensure a good relationship with SST- and pressure-based indices. Areas were selected that represent the Maritime Continent (MC) (100 N - 100S; 900 E - 1500 E) and central to ... eastern Pacific (P) (100 N - 100 S; 1600 E - 1000 W). These regions capture the largest precipitation anomalies associated with the interannual variations of the Walker circulation and contain the largest correlations between GPCP and Nino 3.4 and SOI. Within P and MC the absolute magnitude of the largest correlation is over +0.6.

Because of the spatially varying nature of rainfall, it was decided to use a moving block average which would capture the strongest zonal gradients within the equatorial Pacific. This procedure is unlike many fixed area average indices, and allows for a realistic meridional component of the precipitation gradient and migration of the ascending and descending branches of the Walker circulation.